The Westerbork HI survey of spiral and irregular galaxies. I. HI imaging of late-type dwarf galaxies

Neutral hydrogen observations with the Westerbork Synthesis Radio Telescope are presented for a sample of 73 late-type dwarf galaxies. These observations are part of the WHISP project (Westerbork H I Survey of Spiral and Irregular Galaxies). Here we present H I maps, velocity fields, global profiles and radial surface density profiles of H I, as well as H I masses, H I radii and line widths. For the late-type galaxies in our sample, we find that the ratio of H I extent to optical diameter, defined as 6.4 disk scale lengths, is on average 1.8 +/- 0.8, similar to that seen in spiral galaxies. Most of the dwarf galaxies in this sample are rich in H I, with a typical M_{ion {H}i}/L_B of 1.5. The relative H I content M_ion {HI}/L_R increases towards fainter absolute magnitudes and towards fainter surface brightnesses. Dwarf galaxies with lower average H I column densities also have lower average optical surface brightnesses. We find that lopsidedness is as common among dwarf galaxies as it is in spiral galaxies. About half of the dwarf galaxies in our sample have asymmetric global profiles, a third has a lopsided H I distribution, and about half shows signs of kinematic lopsidedness.